1. Field of the Invention
The present invention relates to a fuel evaporation control system for use in an automobile vehicle, and more particularly to a device for detecting deterioration of a canister used in the fuel evaporation control system.
2. Description of Related Art
A fuel evaporation control system in which gaseous fuel evaporated from a fuel tank of an automobile vehicle is absorbed by a canister and the absorbed fuel is purged into an intake pipe of an engine is known hitherto. The canister is composed of activated charcoal for absorbing the evaporated fuel and a filter for removing dusts contained in the atmospheric air. The canister is connected to a fuel tank, the intake pipe and the atmosphere through respective passages. An amount of fuel purged into the intake pipe is controlled by a control valve installed between the canister and the intake pipe.
JP-A-6-506514 and JP-A-4-265457 disclose such a fuel evaporation control system, and more particularly a technique for detecting an increase in a flow resistance in the canister. In the system disclosed in JP-A-6-506514, a negative pressure is introduced in the fuel evaporation control system, and then a purge control valve is closed. Under this situation, an atmospheric pressure and an in-tank pressure are measured. Then, a difference between both pressures is compared with a predetermined criterion value. If the pressure difference exceeds the criterion value for a predetermined period of time, it is determined that the flow resistance through the canister is abnormally increased. In other words, it is determined that the canister is deteriorated.
However, in the technique disclosed, the criterion value for determining the canister deterioration is set based on various parameters such as engine speed, an engine load, a duty ratio of the purge control valve. Accordingly, a complex process is required in setting the criterion value. Also, it is difficult to precisely determine the canister deterioration because an amount of fuel vaporizing from the fuel tank differs depending on driving conditions of the engine. Further, in the systems disclosed in both JP-A-6-506514 and JP-A-4-265457, deterioration of fuel-absorbing ability of the canister cannot be detected.